PAPER PRODUCT

Abstract

The invention relates to a supercalendered paper product comprising first cellu-losic fibres and a gypsum-fibre composite product as a filler pigment or coating pigment, wherein the gypsum in the gypsum-fibre composite product appears as crystals on the surface of the fibre, and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous second fibre suspension. The invention also relates to a process for producing a supercalendered paper product.

Description

FIELD OF THE INVENTION

The invention relates to a supercalendered paper product including a gypsum-fibre composite product as a coating pigment or a filler pigment. The invention also relates to a process for producing a supercalendered paper product and to the use of gypsum-fibre composite product as a coating pigment or a filler pigment in the production of a supercalendered paper product.

BACKGROUND OF THE INVENTION

A papermaking process starts with stock preparation where cellulosic fibers are mixed with water and mineral filler (usually clay or calcium carbonate or also gypsum). The obtained slurry is delivered by means of a head box on a forming fabric or press fabric or wire to form a fibrous web of cellulosic fibers at the forming section of the paper machine. Then water is drained in the draining section and the formed web is conducted to the press section including a series of roll presses where additional water is removed. The web is then conducted to the drying section of the paper machine where most of the remaining water is evaporated typically by means of steam-heated dryer drums. Post drying operations include calendering where the dry paper product passes between rolls under pressure, thereby improving the surface smoothness and gloss and making the caliper/thickness profile more uniform. There are various calenders such as machine calenders where the rolls usually are steel rolls and include a heated roll (thermo roll), and supercalenders that use alternate hard and soft, heated rolls.

A supercalender is a stack of alternating hard and soft rolls through which paper is passed to increase its density, smoothness and gloss.

Gypsum or calcium sulphate dihydrate CaSO₄.2H₂O is suitable as material for both coating pigment and filler, especially in paper products. Especially good coating pigment and filler is obtained if the particular gypsum has high brightness, gloss and opacity. The gloss is high when the particles are sufficiently small, flat and broad (platy). The opacity is high when the particles are refractive, small and of equal size (narrow particle size distribution).

The morphology of the gypsum product particles can be established by examining scanning electron micrographs. Useful micrographs are obtained e.g. with a scanning electron microscope of the type Philips FEI XL 30 FEG.

The size of the gypsum product particles is expressed as the weight average diameter D₅₀ of the particles contained therein. More precisely, D₅₀ is the diameter of the presumably round particle, smaller than which particles constitute 50% of the total particle weight. D₅₀ can be measured with appropriate devices such as by microscopy.

The flatness of a crystal means that it is thin. The form of flat crystals is suitably expressed by means of the shape ratio SR. The SR is the ratio of the crystal length (the longest measure) to the crystal thickness (the shortest transverse measure). By the SR of the gypsum product is meant the average SR of its individual crystals.

The platyness of a crystal means that it is broad. Platyness is suitable expressed by means of the aspect ratio AR. The AR is the ratio between the crystal length (the longest measure) and the crystal width (the longest transverse measure). By the AR of the gypsum product is meant the average AR of its individual crystals.

Both the SR and the AR of the gypsum product can be estimated by examining its scanning electron micrographs. A suitable scanning electron microscope is the above mentioned Philips FEI XL 30 FEG.

Equal crystal particle size means that the crystal particle size distribution is narrow. The width is expressed as the gravimetric weight distribution WPSD and it is expressed as (D₇₅−D₂₅)/D₅₀ wherein D₇₅, D₂₅ and D₅₀ are the diameters of the presumably round particles, smaller than which particles constitute 75, 25 and 50%, respectively, of the total weight of the particles. The width of the particle distribution is obtained with a suitable particle size analyzer such as the above mentioned type Sedigraph 5100.

Gypsum occurs as a natural mineral or it is formed as a by-product of chemical processes, e.g. as phosphogypsum or flue gas gypsum. In order to refine the gypsum further by crystallising it into coating pigment or filler, it must first be calcined into calcium sulphate hemihydrate (CaSO₄.½H₂O), after which it may be hydrated back by dissolving the hemihydrate in water and precipitating to give pure gypsum. Calcium sulphate may also occur in the form of anhydrite lacking crystalline water (CaSO₄).

Depending on the calcination conditions of the gypsum raw material, the calcium sulphate hemihydrate may occur in two forms; as α- and β-hemihydrate. The β-form is obtained by heat-treating the gypsum raw material at atmospheric pressure while the α-form is obtained by treating the gypsum raw material at a steam pressure which is higher than atmospheric pressure or by means of chemical wet calcination from salt or acid solutions at e.g. about 45° C.

WO 88/05423 discloses a process for the preparation of gypsum by hydrating calcium sulphate hemihydrate in an aqueous slurry thereof, the dry matter content of which is between 20 and 25% by weight. Gypsum is obtained, the largest measure of which is from 100 to 450 μm and the second largest measure of which is from 10 to 40 μm.

AU 620857 (EP 0334292 A1) discloses a process for the preparation of gypsum from a slurry containing not more than 33.33% by weight of ground hemihydrate, thereby yielding needle-like crystals having an average size of between 2 and 200 μm and an aspect ratio between 5 and 50. See page 15, lines 5 to 11, and the examples of this document.

US 2004/0241082 describes a process for the preparation of small needle-like gypsum crystals (length from 5 to 35 μm, width from 1 to 5 μm) from an aqueous slurry of hemihydrate having a dry matter content of between 5 and 25% by weight. The idea in this US document is to reduce the water solubility of the gypsum by means of an additive in order to prevent the crystals from dissolving during paper manufacture.

DE 32 23 178 C1 discloses a process for producing organic fibres coated with one or more mineral substances. One embodiment comprises mixing cellulose fibres, gypsum and water. The mixture is compacted to give a plastic mass which subsequently is dried and mechanically comminuted to give fine particles. The obtained product can be used as an additive or filler e.g. in bitumen masses or putties.

WO 2008/092990 discloses a gypsum product consisting of intact crystals having a size from 0.1 to 2.0 μm. The crystals have a shape ratio SR of at least 2.0, preferably between 2.0 and 50, and an aspect ratio AR between 1.0 and 10, preferably between 1.0 and below 5.0.

WO 2008/092991 discloses a process for the preparation of a gypsum product wherein calcium sulphate hemihydrate and/or calcium sulphate anhydrite and water are contacted so that the calcium sulphate hemihydrate and/or calcium sulphate anhydrite and the water react with each other and form a crystalline gypsum product. The formed reaction mixture has a dry matter content of between 34 and 84% by weight.

WO 2007/003697 discloses a method for coating cellulose particles, produced from dissolved cellulose by precipitation, by contacting the cellulose particles with a light scattering material, followed by the attachment of the light scattering material on the surface of the cellulose particles. The size of the cellulose particles is between 0.05 and 10 μm. The light scattering material comprises silica, silicate, PCC, gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium sulphate or zinc oxide. The coated cellulose particles may be used as a filler or coating pigment of paper or board.

DESCRIPTION OF THE INVENTION

The aim of the invention is to provide a supercalendered paper product having improved properties, such as high brightness, high whiteness, low yellowness, high light scattering, high opacity, low roughness, high gloss and high density.

According to the present invention it was found that a particular gypsum-fibre composite product wherein the gypsum is crystallized on the surface of the fibre and attached fairly strongly to the fibre can be used as a filler pigment or coating pigment in the production of a supercalendered paper product resulting in unexpected improvements in respect of paper properties, such as high brightness, high whiteness, low yellowness, high light scattering, high opacity, low roughness, high gloss and high density. In the production of supercalendered paper also improved retention of the filler pigment and homogenous filler distribution can be obtained. Also higher filler load can be obtained.

Thus, according to a first aspect of the invention there is provided a supercalendered paper product comprising first cellulosic fibres and a gypsum-fibre composite product as a filler pigment or coating pigment, wherein the gypsum in the gypsum-fibre composite product appears as crystals on the surface of the fibre, and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous second fibre suspension.

The gypsum-fibre composite product may be similar to the one described in Finnish patent application FI 20085767 filed on 11 Aug. 2008.

The gypsum is attached to the fibre and consequently the gypsum-fibre composite is shown by most measurement methods as a single piece. The shape and size of the gypsum can roughly be estimated by means of microscopic images. The gypsum crystals attached to the fibre can have the shapes and sizes described in WO 2008/092990 and WO 2008/092991. However, according to the invention the crystallized gypsum can also be needle-like.

The size of the gypsum crystals formed on the surface of the fibre is preferably from 0.1 to 5.0 μm, more preferably from 0.1 to 4.0 μm, and most preferably from 0.2 to 4.0 μm However, in the finished supercalendered paper product the size of the gypsum crystals may be increased.

Preferably the first cellulosic fibres comprise conventional papermaking pulp fibres including chemical, mechanical, chemi-mechanical or deinked pulp fibres. Chemical pulps include kraft pulp and sulphite pulp. Mechanical pulps include stone groundwood pulp (SGW), refiner mechanical pulp (RMP), pressure groundwood (PGW), thermomechanical pulp (TMP), and also chemically treated high-yield pulps such as chemithermomechanical pulp (CTMP). Deinked pulp can be made using mixed office waste (MOW), newsprint (ONP), magazines (OMG) etc. Also mixtures of different pulps can be used.

The average length of the first cellulosic fibre is preferably between 0.5 and 5 mm. Cellulosic fibres derived from softwood typically have an average length of between 1 and 5 mm, preferably between 2 and 4 mm. Cellulosic fibres derived from hardwood typically have an average length of between 0.5 and 3 mm, preferably between 1 and 2 mm.

Preferably the second fibre of the gypsum-fibre composite product comprises a second cellulosic fibre such as a chemical, mechanical, chemi-mechanical or deinked pulp fibre or a synthetic fibre, such as a polyolefine, e.g. polypropene. Chemical pulps include kraft pulp and sulphite pulp. Mechanical pulps include stone groundwood pulp (SGW), refiner mechanical pulp (RMP), pressure groundwood (PGW), thermomechanical pulp (TMP), and also chemically treated high-yield pulps such as chemithermomechanical pulp (CTMP). Deinked pulp can be made using mixed office waste (MOW), newsprint (ONP), magazines (OMG) etc. Also mixtures of different pulps can be used.

The average length of the second fibre is preferably between 0.5 and 5 mm. Cellulosic fibres derived from softwood typically have an average length of between 1 and 5 mm, preferably between 2 and 4 mm. Cellulosic fibres derived from hardwood typically have an average length of between 0.5 and 3 mm, preferably between 1 and 2 mm.

Said first cellulosic fibre and said second cellulosic fibre may be similar or different, preferably similar.

Preferably the weight ratio of gypsum to fibre in the gypsum-fibre composite product on dry basis is in the range from 95:5 to 50:50, preferably from 75:25 to 50:50.

According to the invention the supercalendered paper product may additionally comprise one or more of following substances: a natural or synthetic polymer binder, an optical brightener, a rheology modifier and a sizing agent. These substances or some of these substances may be introduced into the gypsum-fibre composite product. The sizing agent may be a rosin size or a reactive size such as alkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA).

The amount of the gypsum-fibre composite product in the supercalendered paper product is preferably from 10 to 60%, more preferably from 20 to 50% by weight on dry basis. Correspondingly the amount of the first cellulosic fibres in the paper product is preferably from 40 to 90%, more preferably from 50 to 80% by weight on dry basis.

When used as a coating pigment, the gypsum-fibre composite product comprises gypsum crystals preferably having a size of between 0.1 and 1.0 μm, more preferably between 0.5 and 1.0 μm. When used as a filler, the gypsum-fibre composite product comprises gypsum crystals preferably having a size of between 1.0 and 5.0 μm, more preferably between 1.0 and 4.0 μm. As was stated above the size of the gypsum crystals in the finished supercalendered paper product may be increased.

Preferably the weigh ratio of calcium sulphate hemihydrate and/or calcium sulphate anhydrite to water in the crystallization is in the range from 0.03 to 0.6:1, more preferably from 0.05 to 0.5:1.

Preferably the dry fiber content in the crystallization is from 3 to 30% by weight.

Preferably the content of calcium sulphate hemihydrate and/or calcium sulphate anhydrite in the crystallization is from 10 to 57% by weight.

The obtained gypsum-fibre composite product may additionally be homogenized to form a homogenized product or dried and comminuted to form a gypsum-fibre composite product in the form of dry particles.

The second fibre may be comminuted before crystallizing the gypsum thereon. However, it is more preferred to comminute the gypsum-fibre composite product.

The gypsum-fibre composite product may be prepared at a pulp mill or in situ at a paper mill. In the latter case the gypsum-fibre composite product requires a retention time of preferably at least 15 minutes.

A fixative can be introduced into the crystallization.

The fixative can be selected from the group consisting of poly aluminum chloride, poly diallyldimethylammonium chloride (poly DADMAC), anionic and cationic poly-acrylates.

The crystallization can be carried out in the absence of crystallization habit modifiers.

The crystallization can also be carried out in the presence of a crystallization habit modifier.

The crystallization habit modifier can be added to water or aqueous fibre suspension before the calcium sulphate hemihydrate and/or calcium sulphate anhydrite.

The temperature of the water in the reaction mixture can be anything between 0 and 100° C. Preferably, the temperature is between 0 and 80° C., more preferably between 0 and 50° C., even more preferably between 0 and 40° C., most preferably between 0 and 25° C.

The crystallization habit modifier may be an inorganic acid, oxide, base or salt. Examples of useful inorganic oxides, bases and salts are AlF₃, Al₂(SO₄)₃, CaCl₂, Ca(OH)₂, H₃BO₄, NaCl, Na₂SO₄, NaOH, NH₄OH, (NH₄)₂SO₄, MgCl₂, MgSO₄ and MgO.

The crystallization habit modifier may also be an organic compound, which is an alcohol, an acid or a salt. Suitable alcohols are methanol, ethanol, 1-butanol, 2-butanol, 1-hexanol, 2-octanol, glycerol, i-propanol and alkyl polyglucoside based C₈-C₁₀-fatty alcohols.

The crystallization habit modifier is preferably a compound having in its molecule one or several carboxylic or sulphonic acidic groups, or a salt of such a compound. Among the organic acids may be mentioned carboxylic acids such as acetic acid, propionic acid, succinic acid, citric acid, tartaric acid, ethylene diamine succinic acid (EDDS), iminodisuccinic acid (ISA), ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), N-bis-(2-(1,2-dicarboxyethoxy)ethyl aspartic acid (AES), and sulphonic acids such as amino-1-naphthol-3,6-disulphonic acid, 8-amino-1-naphthol-3,6-disulphonic acid, 2-aminophenol-4-sulphonic acid, anthrachinone-2,6-disulphonic acid, 2-mercaptoethanesulphonic acid, poly(styrene sulphonic acid), poly(vinylsulphonic acid), as well as the di-, tetra- and hexa-aminostilbenesulfonic acids.

Among the organic salt may be mentioned the salts of carboxylic acids such as Mg formiate, Na- and NH₄-acetate, Na₂-maleate, NH₄-citrate, Na₂-succinate, K-oleate, K-stearate, Na₂-ethylenediamine tetraacetic acid (Na₂-EDTA), Na₆-aspartamic acid ethoxy succinate (Na₆-AES) and Na₆-aminotriethoxy succinate (Na₆-TCA).

Also the salt of sulphonic acids are useful, such as Na-n-(C₁₀-C₁₃)-alkylbenzene sulphonate, C₁₀-C₁₆-alkylbenzene sulphonate, Na-1-octyl sulphonate, Na-1-dodecane sulphonate, Na-1-hexadecane sulphonate, the K-fatty acid sulphonates, the Na—C₁₄-C₁₆-olefin sulphonate, the Na-alkylnaphthalene sulphonates with anionic or non-ionic surfactants, di-K-oleic acid sulphonates, as well as the salts of di-, tetra-, and hexaaminostilbene sulphonic acids. Among organic salts containing sulphur should also be mentioned the sulphates such as the C₁₂-C₁₄-fatty alcohol ether sulphates, Na-2-ethyl hexyl sulphate, Na-n-dodecyl sulphate and Na-lauryl sulphate, and the sulphosuccinates such as the monoalkyl polyglycol ether of Na-sulphosuccinate, Na-dioctyl sulphosuccinate and Na-dialkyl sulphosuccinate.

Phosphates may also be used, such as the Na-nonylphenyl- and Na-dinonyl phenylethoxylated phosphate esters, the K-aryl ether phosphates, as well as the triethanolamine salts of polyaryl polyetherphosphate.

As crystallization habit modifier may also be used cationic surfactants such as octyl amine, triethanol amine, di(hydrogenated animal fat alkyl)dimethyl ammonium chloride, and non-ionic surfactants such as a variety of modified fatty alcohol ethoxylates. Among useful polymeric acids, salts, amides and alcohols may be mentioned the polyacrylic acids and polyacrylates, the acrylate-maleate copolymers, polyacrylamide, poly(2-ethyl-2-oxazoline), polyvinyl phosphonic acid, the copolymer of acrylic acid and allylhydroxypropyl sulphonate (AA-AHPS), poly-α-hydroxyacrylic acid (PHAS), polyvinyl alcohol, and poly(methyl vinyl ether-alt.-maleic acid).

Especially preferable crystallization habit modifiers are ethylene diamine succinic acid (EDDS), iminodisuccinic acid (ISA), ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), N-bis-(2-(1,2-dicarboxyethoxy)ethyl aspartic acid (AES), the di-, tetra- and hexa-aminostilbenesulfonic acids and their salts such as Na-aminotriethoxy succinate (Na₆-TCA), as well as the alkylbenzenesulphonates.

The crystallization habit modifier can be used in an amount of 0.01 to 5.0%, most preferably 0.02-1.78%, based on the weight of the calcium sulphate hemihydrate and/or calcium sulphate anhydrite.

In the crystallization β-calcium sulphate hemihydrate is typically used. It may be prepared by heating gypsum raw-material to a temperature of between 140 and 300° C., preferably from 150 to 200° C. At lower temperatures, the gypsum raw-material is not sufficiently dehydrated and at higher temperatures it is over-dehydrated into anhydrite. Calcinated calcium sulphate hemihydrate usually contains impurities in the form of small amounts of calcium sulphate dihydrate and/or calcium sulphate anhydrite. It is preferable to use β-calcium sulphate hemihydrate obtained by flash calcination, e.g. by fluid bed calcination, whereby the gypsum raw-material is heated to the required temperature as fast as possible. However, it is also possible to use α-calcium sulphate hemihydrate in the crystallization.

It is also possible to use calcium sulphate anhydrite as starting. The anhydrite is obtained by calcination of gypsum raw material. There are three forms of anhydrite; the first one, the so called Anhydrite I, is unable to form gypsum by reaction with water like the insoluble Anhydrites II-u and II-E. The other forms, the so called Anhydrite III, also known as soluble anhydrite has three forms: β-anhydrite III, β-anhydrite III′, and α-anhydrite III and Anhydrite II-s form pure gypsum upon contact with water.

After the calcium sulphate hemihydrate and/or calcium sulphate anhydrite, aqueous fibre suspension and optionally crystallization habit modifier have been contacted, they are allowed to react into calcium sulphate dihydrate i.e. gypsum. The reaction takes e.g. place by mixing, preferably by mixing strongly, said substances together for a sufficient period of time, which can easily be determined experimentally. At high dry matter contents strong mixing is necessary because, the slurry is thick and the reagents do not easily come into contact with each other. Preferably the hemihydrate and/or anhydrite, the aqueous fibre suspension and optionally the crystallization habit modifier are mixed at the above mentioned temperature given for the water. The initial pH is typically acidic, preferably between 3 and 7, more preferably between 3 and 6. If necessary, the pH is regulated by means of an aqueous solution of NaOH and/or H₂SO₄, typically a 10% solution of NaOH and/or H₂SO₄.

Because gypsum has a lower solubility in water than hemihydrate and soluble anhydrite, the gypsum formed by the reaction of hemihydrate and/or anhydrite with water immediately tends to crystallize from the water medium on the second fibre. The crystallization can be regulated by means of the above mentioned crystallization habit modifier so that the useful gypsum-fibre composite product is obtained.

The gypsum-fibre composite product can also be treated with other additives. A typical additive is a biocide which prevents the activity of microorganisms when storing and using the product.

According to a second aspect of the invention there is provided a process for producing a supercalendered paper product as defined above comprising providing a stock where first cellulosic fibers are mixed with water and the gypsum-fibre composite product, the obtained slurry is delivered to the forming section of a paper machine, then water is drained in the draining section and the formed web is conducted to the press section where additional water is removed, then the web is conducted to the drying section where most of the remaining water is evaporated, and finally supercalendering the paper to improve the surface smoothness and gloss of the paper.

The gypsum-fibre composite product may be introduced in the form of an aqueous product or in a dried form, optionally comminuted into the form of particles.

The supercalendering of the paper may be carried out in an on-line calender or in an off-line calender. In the latter case the calender is separate from the actual paper machine. The rolls of the supercalenders may be heated thermo rolls.

Additionally the invention relates to the use of a gypsum-fibre composite product wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous fibre suspension, as a filler pigment or coating pigment in the production of a supercalendered paper product.

Preferably the amount of the gypsum-fibre composite product in the supercalendered paper product is from 10 to 60%, preferably from 20 to 50% by weight on dry basis. Correspondingly the amount of said cellulosic fibres in the paper product is preferably from 40 to 90%, more preferably from 50 to 80% by weight on dry basis.

SHORT DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 show electron microscope micrographs of calcium sulfate dihydrate-fiber composite products of examples 1-8, and FIGS. 9-17 show various properties of supercalendered (SC) paper samples wherein the filler is a calcium sulfate dihydrate-fiber composite product, precipitated calcium sulphate (PCS) or kaolin clay.

FIG. 1a shows SEM micrograph of calcium sulfate dihydrate/TMP composite at hemihydrate solids content of 18% (HH/(HH+water)),

FIG. 1b shows SEM micrograph of the same composite as in FIG. 1a washed in saturated calcium sulfate solution,

FIG. 2a shows SEM micrograph of calcium sulfate dihydrate/TMP composite at hemihydrate solids content of 42% (HH/(HH+water)),

FIG. 2b shows SEM micrograph of the same composite as in FIG. 2a washed in saturated calcium sulfate solution,

FIG. 3 shows SEM micrograph of calcium sulfate dihydrate/eucalyptus kraft pulp composite at hemihydrate solids content of 6.25% (HH/(HH+water)),

FIG. 4 shows SEM micrograph of the fibers from calcium sulfate dihydrate/eucalyptus kraft pulp composite at hemihydrate solids content of 7.5% (HH/(HH+water)),

FIG. 5a shows SEM micrograph of calcium sulfate dihydrate/pine kraft pulp composite using poly aluminum chloride as fixative, composite being washed with saturated calcium sulfate solution,

FIG. 5b shows SEM micrograph of the same composite as in FIG. 5a being stirred with Heidolph laboratory mixer at 350 rpm for a couple of minutes,

FIG. 6a shows SEM micrograph of calcium sulfate dihydrate/pine kraft pulp composite using poly-DADMAC as fixative, composite being washed with saturated calcium sulfate solution,

FIG. 6b shows SEM micrograph of the same composite as in FIG. 6a being stirred with Heidolph laboratory mixer at 350 rpm for a couple of minutes,

FIG. 7 shows SEM micrograph of calcium sulfate dihydrate/birch kraft pulp composite washed with saturated calcium sulfate solution,

FIG. 8 shows SEM micrograph of calcium sulfate dihydrate/plastic fiber composite washed with saturated calcium sulfate solution,

FIG. 9 shows ISO Brightness of SC paper samples,

FIG. 10 shows CIE Whiteness of SC paper samples,

FIG. 11 shows Yellowness of SC paper samples,

FIG. 12 shows Light scattering of SC paper samples,

FIG. 13 shows Opacity of SC paper samples,

FIG. 14 shows PPS Roughness of SC paper samples,

FIG. 15 shows Gloss of SC paper samples,

FIG. 16 shows Gloss vs. paper bulk of SC paper samples, and

FIG. 17 shows Air permeability (Bendtsen porosity) of SC paper samples.

EXAMPLES

In the following the invention will be illustrated in more detail by means of examples. The purpose of the examples is not to restrict the scope of the claims. In this specification the percentages refer to % by weight unless otherwise specified.

First, general information about the syntheses and product analyses is disclosed. Then, data about each example is presented.

Synthesis

General information is first presented. A method optimization for the paper pigments was carried out. The parameters were:

HH (initial hemihydrate, w-%)    5-57
Fiber concentration (w-%)        3-30
Additive concentration (w-% of DH(dihydrate)) 0.100-1

The reaction was carried out at system pH. The amount of habit modifier chemical is calculated as percent of the precipitated calcium sulfate dihydrate (w-% of DH)

The experiments were performed with the following equipment.

The reactor was of Hobart type N50CE. The hemihydrate and the chemicals are added batchwise to the aqueous fiber suspension phase and a hemihydrate slurry with an initial solids of 5-57 w-% is obtained. Mixing speed is about 250-500 rpm. Reaction is carried out at system pH.

Analysis

Morphology of calcium sulfate dihydrate was studied by using FEI XL 30 FEG scanning electron microscope. Conversion of hemihydrate to dihydrate was analyzed using Mettler Toledo TGA/SDTA85 1/1100-thermogravimetric analyzer (TG). Crystal structure was determined with Philips X'pert x-ray powder diffractometer (XRD).

Example 1

1. 800 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of drops of biocide (Fennosan IT 21) is added.
2. 200 g of TMP (Thermomechanical pulp) with solids content of 36% is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 200 g (giving 18% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite is shown in FIG. 1a, after washing with calcium sulfate saturated water in FIG. 1b.

Example 2

1. 430 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of drops of biocide (Fennosan IT 21) is added.
2. 570 g of TMP (Thermomechanical pulp) with solids content of 36% is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 570 g (giving 42% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite is shown in FIG. 2a, after washing with calcium sulfate saturated water in FIG. 2b.

Example 3

1. 456.5 g of eucalyptus kraft pulp with solids content of 17.7% is placed into the Hobart N50 CE laboratory mixer.
2. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 25 g (giving 6.25% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
3. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite after washing with calcium sulfate saturated water is shown in FIG. 3.

Example 4

1. 47 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of drops of biocide (Fennosan IT 21) is added.
2. 295.5 g of eucalyptus kraft pulp with solids content of 17.7% is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 25 g (giving 7.5% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained fiber product is shown in FIG. 4.

Example 5

1. 44.8 g of water is placed into the Hobart N50 CE laboratory mixer. 1.6 g of poly aluminum chloride and couple of drops of biocide (Fennosan IT 21) is added.
2. 640 g of pine kraft pulp with solids content of 7% is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 160 g (giving 20% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite after washing with calcium sulfate saturated water is shown in FIG. 5.

Example 6

1. 15.8 g of water is placed into the Hobart N50 CE laboratory mixer. 0.6 g of poly DADMAC and couple of drops of biocide (Fennosan IT 21) is added.
2. 226 g of pine kraft pulp with solids content of 7% is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 300 g (giving 57% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite after washing with calcium sulfate saturated water is shown in FIG. 6.

Example 7

1. 116 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of drops of biocide (Fennosan IT 21) is added.
2. 800 g of birch kraft pulp (solids content 14.5%) is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 200 g (giving 20% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite after washing with calcium sulfate saturated water is shown in FIG. 7.

Example 8

1. 600 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of drops of biocide (Fennosan IT 21) is added.
2. 10 g of synthetic polypropene fiber is added to the reactor.
3. Fluidized bed calcined β-calcium sulphate hemihydrate is evenly added to the reactor with the operation speed of the stirrer set to position 1. The total amount of hemihydrate added is 300 g (giving 34% by weight of HH/(HH+water)). After the addition the operation speed of the stirrer is raised to position 2. Composite is stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.

The obtained pigment-fiber composite after washing with calcium sulfate saturated water is shown in FIG. 8.

Example 9

Pigment-filler composite was prepared as described in Example 1 by using thermo-mechanical pulp (TMP) at fiber solids content of 8%. Calcined β-calcium sulphate hemihydrate was added in an amount giving 20% of HH/(HH+water).

Application test on uncoated supercalendered paper was carried as follows.

Composite samples were disintegrated using Hollander refiner. In sheet making the composite was mixed with untreated fiber fraction and compared with traditional kaolin and PCS (precipitated calcium sulphate) fillers. The used filler level was 30% (for the composite the filler level refers to the gypsum level), and the target basis weight was 60±2 g/m². Filler level was adjusted by changing the ratio between composite and untreated fiber. Samples were calendered using 2+2 nips at line loads of 100, 175 and 250 kN/m. Roll temperature was 80° C., relative humidity 85%, and calendaring speed 30 m/min.

The ISO-Brightness of paper samples was measured at R₄₅₇. The results are shown in FIG. 9. The results show that the effect of PCS and the Composite of the present invention in paper brightness was similar, whereas the kaolin clay gave a brightness of about 4% units lower than for PCS and Composite.

The CIE Whiteness of paper samples was measured. The results are shown in FIG. 10. The results show that the filler-fiber composite gave significantly higher whiteness to the paper than PCS and kaolin clay.

The Yellowness (CIE yellow colour coordinate b*(C/2°)) of paper samples was measured. The results are shown in FIG. 11. The results show that the filler-fiber composite gave significantly lower yellowness values than PCS and kaolin clay.

The Light scattering of paper samples was measured. The results are shown in FIG. 12. The results show that the filler-fiber composite improved light scattering with about 5-10 units compared to PCS and kaolin clay.

The Opacity of paper samples was measured. The results are shown in FIG. 13. From the results it can be concluded that improvement in light scattering had also positive impact on the opacity values. At lower line loads the filler-fiber composite showed about one unit higher opacity than kaolin and about 2 units higher than PCS. The differences increased at higher line loads being about 2 and 3 units higher than kaolin and PCS, respectively.

The PPS Roughness of paper samples was measured. The results are shown in FIG. 14. The results show that kaolin and filler-fiber composite had similar roughness while PCS had about 0.1 μm higher roughness. The filler-fiber composite had also lower difference in roughness between top side (TS) and wire side (WS).

The Gloss 75° of paper samples was measured. The results are shown in FIG. 15. The results show that the filler-fiber composite had an about 4 units higher calendered gloss than PCS and one unit higher than kaolin.

In FIG. 16 the calendered gloss is shown against paper bulk. From the results it can be seen that the filler-fiber composite had about 0.1 units higher bulk than kaolin at the same gloss value.

The porosity of paper samples was measured by the Bendtsen method. This method measures the rate at which air will pass through a sheet of paper at a set of pressure. A high porosity indicates the paper allows the air to travel through relatively easy. The results are shown in FIG. 17. The results show that the filler-fiber composite had a lower porosity, i.e. higher density than PCS. Kaolin samples had the highest density.

Claims

1. A supercalendered paper product comprising first cellulosic fibres and a gypsum-fibre composite product as a filler pigment or coating pigment, wherein the gypsum in the gypsum-fibre composite product appears as crystals on the surface of the fibre, and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous second fibre suspension, and wherein the gypsum crystals have a size from 0.1 to 5.0 μm.

2. The supercalendered paper product according to claim 1, wherein the first cellulosic fibres comprise chemical, mechanical, chemi-mechanical or deinked pulp fibres.

3. The supercalendered paper product according to claim 1, wherein the second fibre of the gypsum-fibre composite product comprises a second cellulosic fibre such as a chemical, mechanical, chemi-mechanical or deinked pulp fibre or a synthetic fibre.

4. The supercalendered paper product according to claim 3, wherein average length of the second fibre of the gypsum-fibre composite product is between 0.5 and 5 mm.

5. The supercalendered paper product according to claim 1, wherein the weight ratio of gypsum to fibre in the gypsum-fibre composite product on dry basis is in the range from 95:5 to 50:50.

6. The supercalendered paper product according claim 1 additionally comprising one or more of following substances: a natural or synthetic polymer binder, an optical brightener, a rheology modifier and a sizing agent.

7. The supercalendered paper product according to claim 1, wherein the amount of the gypsum-fibre composite product is from 10 to 60%, by weight on dry basis.

8. The supercalendered paper product according to claim 1, wherein in the crystallization the calcium sulphate hemihydrate comprises α-calcium sulphate hemihydrate or β-calcium sulphate hemihydrate.

9. The supercalendered paper product according to claim 1, wherein in the crystallization the weight ratio of calcium sulphate hemihydrate and/or calcium sulphate anhydrite to water is in the range from 0.03 to 0.6:1, preferably from 0.05 to 0.5:1.

10. The supercalendered paper product according to claim 1, wherein in the crystallization the dry second fiber content is from 3 to 30% by weight.

11. The supercalendered paper product according to claim 1, wherein in the crystallization the content of calcium sulphate hemihydrate and/or calcium sulphate anhydrite is from 5 to 57% by weight.

12. A process for producing a supercalendered paper product as defined in claim 1 comprising providing a stock where first cellulosic fibers are mixed with water and the gypsum-fibre composite product, the obtained slurry is delivered to the forming section of a paper machine, then water is drained in the draining section and the formed web is conducted to the press section where additional water is removed, then the web is conducted to the drying section where most of the remaining water is evaporated, and finally supercalendering the paper to improve the surface smoothness and gloss of the paper.

13. The process of claim 12, wherein the gypsum-fibre composite product is introduced in the form of an aqueous product or in a dried form, optionally comminuted into the form of particles.

14. Use of a gypsum-fibre composite product wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous fibre suspension, and wherein the gypsum crystals have a size from 0.1 to 5.0 μm, as a filler pigment or coating pigment in the production of a supercalendered paper product.

15. Use according to claim 14, wherein the amount of the gypsum-fibre composite product is from 10 to 60%, by weight on dry basis of the supercalendered paper product.